1. Field of the Invention
The present invention relates to unbalanced-to-balanced converters (baluns), and more particularly, to an unbalanced-to-balanced converter used which is usable as a 180-degree phase shifter which divides one signal into two signals having phases 180 degrees apart and having the same level.
2. Description of the Related Art
FIG. 2 is a conventional unbalanced-to-balanced converter. In FIG. 2, an unbalanced-to-balanced converter 20 includes three microstriplines 21, 22, and 23 serving as first, second, and third distributed-constant lines disposed sufficiently close to each other that they are coupled with each other, a signal input terminal 24, and signal output terminals 25 and 26.
In FIG. 2, the signal input terminal 24 is connected to the left-hand end of the second microstripline serving as a signal input end, a signal output terminal 25 is connected to the right-hand end of the second microstripline serving as a first signal output end, and a signal output terminal 26 is connected to the right-hand end of the third microstripline serving as a second signal output end. The right-hand ends of the first and second microstriplines are connected to each other, and the left-hand ends of the first and the third microstriplines are grounded.
The microstriplines 21, 22, and 23 are designed such that their length equals one fourth the wavelength of a signal to be used.
In the unbalanced-to-balanced converter 20 configured as described above, when a signal is input to the signal input terminal 24, a part of the signal passes through the microstripline 22 and is output from the signal output terminal 25, and another part of the signal resonates in the microstripline 21 and generates a standing wave. The power of the standing wave generated in the microstripline 21 is transferred to the microstripline 23 and output from the signal output terminal 26. The microstriplines 21, 22, and 23 are disposed with intervals among them such that a signal output from the signal output terminal 25 has the same level as that output from the signal output terminal 26. The signals output from the signal output terminals 25 and 26 have phases 180 degrees apart. In this way, one signal is divided into two signals having phases 180 degrees apart and the same level.
In the above unbalanced-to-balanced converter 20, however, the distributed-constant line 21 in which the standing wave is generated is disposed away from the distributed-constant line 23 to which the power of the standing wave is transferred, with the distributed-constant line 22 being sandwiched therebetween. Therefore, the power is transferred to the distributed-constant line 23 with low efficiency. To transfer half of the power of an input signal, it is necessary to set the intervals among the distributed-constant lines very small. In this situation, advanced manufacturing technology is required, and the cost of manufacturing and of manufacturing equipment becomes high.